magnetoelectric quasi-(0-3) nanocomposite heterostructures · number), brighter spots on the bfo...

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Received 17 Oct 2014 | Accepted 19 Feb 2015 | Published 26 Mar 2015

Magnetoelectric quasi-(0-3) nanocompositeheterostructuresYanxi Li1,*, Zhongchang Wang2,*, Jianjun Yao1,3,*, Tiannan Yang4, Zhiguang Wang1, Jia-Mian Hu4, Chunlin Chen2,

Rong Sun5, Zhipeng Tian6, Jiefang Li1, Long-Qing Chen4 & Dwight Viehland1

Magnetoelectric composite thin films hold substantial promise for applications in novel

multifunctional devices. However, there are presently shortcomings for both the extensively

studied bilayer epitaxial (2-2) and vertically architectured nanocomposite (1-3) film systems,

restricting their applications. Here we design a novel growth strategy to fabricate an

architectured nanocomposite heterostructure with magnetic quasiparticles (0) embedded in

a ferroelectric film matrix (3) by alternately growing (2-2) and (1-3) layers within the film.

The new heteroepitaxial films not only overcome the clamping effect from substrate, but

also significantly suppress the leakage current paths through the ferromagnetic phase.

We demonstrate, by focusing on switching characteristics of the piezoresponse, that the

heterostructure shows magnetic field dependence of piezoelectricity due to the improved

coupling enabled by good connectivity amongst the piezoelectric and magnetostrictive

phases. This new architectured magnetoelectric heterostructures may open a new avenue for

applications of magnetoelectric films in micro-devices.

DOI: 10.1038/ncomms7680

1 Department of Materials Science and Engineering, Virginia Tech, Blacksburg, Virginia 24061, USA. 2 Advanced Institute for Materials Research, TohokuUniversity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. 3 Asylum Research, Santa Barbara, California 93117, USA. 4 Department of Materials Scienceand Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA. 5 Institute of Engineering Innovation, The University of Tokyo,2-11-16, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan. 6 Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia 24061, USA.* These authors contributed equally to this work. Correspondence and requests for materials should be addressed to Y.X.L. (email: [email protected]) or toZ.C.W (email: [email protected]).

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With the development of new devices miniaturizingtechnology, there is growing interest in combiningelectronic and magnetic properties into multifunc-

tional thin-film materials for potential applications in noveldevices1–4. Magnetoelectric (ME) materials, which would exhibitan induced polarization/magnetization under external magneticfield/electric field, are especially attractive because they not onlybear the properties of individual polar and spin subsystems butalso exhibit additional product functionalities due to the MEinteractions between them5–8. Compared with single-phase MEmaterials, ME composites of piezoelectric and magnetostrictivecomponents are particularly appealing due to their much higherME coefficients and maximum operational temperatures9,10. Forsuch composites, the ME effect is a product property of thepiezoelectric and piezomagnetic coefficient of the correspondingphases11. The average ME properties can be tailored by choosingnot only suitable individual component phases but also compositearchitectures. On the micro- or nano-scale, the current phaseconnectivity architectures, which have been extensively studiedfor the ME composite thin films, can be summarized by twotypes: (i) vertical heteroepitaxial nanocomposites with nanorodsof one phase embedded in a matrix of the other (named as 1-3)(refs 12–16); and (ii) multilayering (named as 2-2) (refs 17–21).However, there are shortcomings for both of these forms: theeffective ME coupling of the (2-2) film connectivity is limited bythe clamping of films to the substrate22, while the low resistivityof the interconnected ferromagnetic phase of the (1-3) structurelowers the net resistivity of the composite layer, which in turnprovides a path for dielectric leakage current.

Here we propose a new concept to overcome the limitation ofboth the (2-2) and (1-3) phase connectivities by alternatelygrowing (2-2) and (1-3) layers within the film, and demonstratethe formation of a new quasi-(0-3) heterostructure with magneticquasiparticles (0) embedded in a ferroelectric film matrix (3).Different from the previously reported (0-3) composites23–25 witha randomly distributed magnetic phase component embedded ina matrix, the designed new structure has a somewhat orderlyarranged magnetic nanorod particles with the same height andapproximate similar planar area as those of the middle layer. Sucha new structure overcomes the clamping effect from the substrateof the (2-2) connectivity, while significantly reducing the leakagecurrent paths through the ferromagnetic phase of the (1-3)structure. As a result, the connectivity amongst the constituentphases shows an improved ME coupling along the verticaldirection due to the improved physical and electrical contactbetween piezoelectric and magnetostrictive phases on all sides,enhancing thereby the potential for ME effects. This opens up anew avenue of approach for the ME heterostructures in micro-device applications.

ResultsPreparation and characterization of the heterostructures. Filmswere grown on (001) oriented SrTiO3 (STO) substrates usingpulsed laser deposition by alternately using targets of pure BiFeO3

and of a 65:35 BiFeO3-CoFe2O4 (BFO-CFO) solid solution.Figure 1a shows the film growth process and a schematic diagramof the as-grown quasi-(0-3) heterostructures, which comprise ofCFO magnetic nanorod particles embedded in a BFO ferroelectricfilm matrix. X-ray diffraction reveals that the nanostructure ishighly orientated on the substrate. Further 2y-o scans show arelationship of BFO (002) // STO (002) between the perovskiteBFO phase in the matrix of the film and the STO substrate(Fig. 1c). A (002) SrRuO3 (SRO) layer, which is used as a bottomelectrode for property measurements, is also closely oriented tothe BFO (002). A single crystal (004) peak for the spinel CFO

nanorod particles in the film is also clearly identified. Figure 1dshows an atomic force microscopy (AFM) image of the surfacemorphology, from which a uniform square BFO grain array isidentified on the surface. The corresponding piezoresponse forcemicroscopy (PFM) images reveal good piezoelectric properties(Fig. 1e,f). The domain structures are not constrained by grainboundaries, revealing good low-angle boundaries between grains.This type of semi-coherent grain boundary will benefit straintransfer between phases in the nanocomposite.

To reveal microstructures of the films, we conducted cross-sectional transmission electron microscopy (TEM) imaging, asshown in Fig. 2. The CFO phase (indicated by arrows in Fig. 2a) isfound to be embedded in the BFO matrix phase in the middle ofthe original BFO–CFO layer in a somewhat ordered fashion andthe BFO bottom and top layers are well connected through theBFO matrix, forming a three-layer quasi-(0-3) heterostructure.With a size of about 500 nm lateral and 300 nm vertical, theseparticles are a little large to be deemed as (0) for those CFOparticles. However, with regard to the film size and consideringthe surrounding matrix that the particles are embedded, it seemsappropriate to refer to this composite phase connectivity as a‘quasi-(0-3) structure’. In 2013, Stratulat et al.26 reported a novelmethod about self-assembly process, which was based onpatterning the nucleation points through a hard mask, to createlarge areas of perfectly ordered film with first phase pillars embedin second phase matrix. Similar to this novel process, we alsoobtained orderly distributed ferromagnetic phase insideferroelectric matrix by simple method. Energy-dispersive X-rayspectroscopy (EDS) mapping identifies chemically the individual(well-aligned) nanorod particles as CFO, as the Bi concentrationis low, while the Co concentration is high (Fig. 2b). In addition,no signals from any impurity elements in the film are identified inthe EDS spectrum, indicating that no significant amounts ofimpurities are introduced during sample preparation. Analysis ofselected-area electron diffraction patterns shows that the nanorodparticles are of single phase with [112] zone axis (SupplementaryFig. 1). Furthermore, the selected-area electron diffractionpatterns taken from a region larger than a nanorod particlereveal two sets of reflections, which belong to the BFO and CFOphases, indicating that no additional third phases form in thelayer. To extract atomic-scale information, we present atomic-resolution high-angle annular dark-field (HAADF) scanningTEM (STEM) images (Fig. 2d,e). Since the intensity of anatomic column is approximately proportional to Z1.7 (Z: atomicnumber), brighter spots on the BFO side represent Bi columns,while darker ones represent Fe ones; whereas on the CFO side,brighter spots are Fe columns that have a double concentrationrelative to other Fe columns that are darker, and the other darkercolumns are Co and O columns. In addition, one can notice thatthe interface is near atomically abrupt and coherent with noamorphous layers, secondary-phase layers, contaminants ortransition regions. This confirms a clean and direct contact ofCFO to BFO at the atomic scale (Fig. 2d). The CFO nanorodparticles maintain the spinel structure without clear evidence ofinter-diffusion, even though they are embedded in the BFOmatrix. These findings are also corroborated by annular bright-field (ABF) STEM images, where the O atomic columns can alsobe identified (Supplementary Fig. 2).

Ferroelectric and ferromagnetic properties. Ferroelectric (P–E)hysteresis measurements (Fig. 3a) at room temperature reveal theferroelectric nature for the BFO phase of the heterostructures.The saturation polarization is Ps¼ 77 mC cm� 2, with a remnantpolarization of Pr¼ 68mC cm� 2 and a coercive field ofEc¼ 33 kV mm� 1. The P–E loops are slightly asymmetric,

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7680

2 NATURE COMMUNICATIONS | 6:6680 | DOI: 10.1038/ncomms7680 | www.nature.com/naturecommunications



implying that an internal bias field exists at the interface betweenelectrode and thin film, that is, the loops can be saturated inforward and reverse directions. These values are considerablyhigher compared with prior studies of the P–E loops for (1-3)BFO–CFO films, which have values of Ps¼ 53 mC cm� 2,

Pr¼ 34 mC cm� 2 and Ec¼ 20 kV mm� 1. Clearly, the ferro-electric properties are altered by the quasi-(0-3) connectivityobtained by pre-depositing and post-depositing bottom and topBFO layers. The heterostructured film has a higher resistivity thanthe two-phase BFO–CFO middle layer, which enables retention of

43 44 45 46 472� (°)

Log

inte

nsity

(C

PS

)

CFO(004)

BFO(002) SRO

(002)

STO(002)

Substrate

Figure 1 | Film growth process and structural characterization. (a,b) Sketch of the film growth process for nanocomposites of CFO spinel pillars inside a

BFO perovskite matrix (a) and of the heterostructure similar to the (0-3) particulate composite (b). (c) 2y-o X-ray scans for the film. (d–f) AFM

topography (d) PFM out-of-plane phase (e) and amplitude (f) images of the film. Scale bar, 1 mm.

SrTiO3

BiFeO3

CoFe2O4

BF Co Fe Bi O

de

BiFeO3

CoFe2O4

BiFeO3

CoFe2O4

SrRuO3

[112] [111]

[220]

Figure 2 | Structural and chemical analysis of the nanocomposite films. (a) A TEM image of the nanocomposite film. The CoFe2O4 phase is indicated by

arrows. Scale bar, 500 nm. (b) A TEM image of the film and corresponding energy-dispersive X-ray spectroscopy (EDS) mapping of the Co, Fe, Bi and O.

(c) Aberration-corrected STEM image of the interfaces in the nanocomposite films viewed from [112] direction. Scale bar, 200 nm. (d,e) Atomic-resolution

HAADF STEM images at the CFO/BFO interface (d) and within the CFO film (e). Scale bar, 1 nm. The locations of the images are highlighted in c.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7680 ARTICLE




charge and allows better measurement of the dielectric displace-ment current of the polarization. Figure 3b shows magnetic M–Hhysteresis loops of the BFO/BFO–CFO/BFO quasi-(0-3) hetero-structures using both in-plane and out-of-plane measurementconfigurations. The magnetization values are normalized to thevolume fraction of the CFO phase in the film. Although theshapes of M–H loops for the in-plane and out-of-plane directionsare found to be similar to those previously reported for singlephase CFO and (1-3) self-assembled BFO–CFO films27–29 withnearly equal coercive field Hc values, the remnant magnetizationMr along the in-plane direction is lower than that along the out-of-plane direction. For the quasi-(0-3) heterostructures, the CFOnanorod particles in the template layer are dimensionallyisolated30. So the spin state is less stable along the in-planedirection than the out-of-plane one, which makes spin rotationeasier. It has been shown that the epitaxial CFO films grown onSTO are compressed along in-plane direction. Since CFO has aconsiderable negative magnetostriction, the magnetoelasticenergy makes a notable contribution to the total energy, whichin turn forces the out-of-plane direction to be the easy axis. Smallresidual strain in the BFO-CFO layers may cause the existing ofanisotropy between the in-plane and out-of-plane directions.Note that the top and bottom BFO layers maintain the strain stateof the middle BFO–CFO layers, imposing insignificant effects onits magnetic properties.

Magnetic field dependence of piezoelectricity. The new het-erostructure may also possess good piezoelectricity and MEproperties. PFM switching characteristic measurement before andafter applying a controllable d.c. magnetic bias confirms thepresence of piezoelectricity and ME coupling in the BFO/BFO–CFO/BFO quasi-(0-3) heterostructures7. A sequence of trianglesquare d.c. voltages pulses31 accompanied with a.c. voltage areused to the sample through the conductive tip for achieving theswitch of the polarization of the BFO phase in theheterostructure. Meanwhile, the corresponding piezoresponse isalso measured. Fig. 4a and b show PFM amplitude and phasesignal, which are the generated characteristic butterfly loops andhysteresis loops, respectively. The amplitude and phase aremeasured using a pulse-on-and-off triangle–square waveform.The advantage for the signal obtained while the pulse is off is thatthe electrostatic effect is pronouncedly reduced32. The averagephase contrast curves of switching with the d.c. magnetic bias andwithout the bias are shown in Fig. 4b, which are found to be closeto 180�, consistent with the phase changed from the polarizationswitch33. The maximum displacement (PFM amplitude) is43 p.m. without external magnetic bias with about 9 V of the

corresponding coercive voltage. In comparison, under a 2900 Oeexternal magnetic bias, the maximum displacement is increasedto 51 p.m. with the enhancement of the corresponding coercivevoltage to 12 V. This demonstrates that electrical properties arechanged by the applied magnetic field, indicative of a MEcoupling in the heterostructure.

To confirm the presence of a ME effect, we conducted multiplemeasurements from many areas of the films. A statisticschart (Fig. 4c) is made for the piezoresponse displacement valuedifference, which is measured at the same points before and afterthe application of external magnetic bias for the various sampleareas. The measured piezoresponse displacement value undermagnetic field (B58.7 p.m. on average) is larger than that withoutmagnetic field (B47.9 p.m. on average). The confidence intervalfor both piezoresponse displacement before/after applied mag-netic field are shown on Supplementary Table 1. Moreover, sincea slight increase in the coercive field Ec is observed after theapplication of external magnetic bias, we provide anotherstatistical chart (Fig. 4d) summarizing the piezoresponsedisplacement and the difference in the Ec value for themeasurement points before and after application of externalmagnetic bias for comparison. One can clearly see that the datafall in two groups: the black points with a relatively smallerpiezoresponse displacement and Ec values before applying themagnetic bias, and the red ones with an enhanced piezoresponsedisplacement and Ec after applying magnetic bias (separated by ablue line). These results confirm the ME effect in the new quasi-(0-3) heterostructures. The switching characteristics of thepiezoresponse of the heterostructures from various sampleregions (with the statistics given) show changes in electricpolarization upon applying magnetic bias.

On the basis of piezoresponse displacement values changingwith the absence/presence of magnetic field, combined with thestatistics method, the lateral ME coefficient a31 for the quasi-(0-3)heterostructures was calculated by following Xie’s approach7

(Fig. 4a). Here, the application of a lateral magnetic field ofnH1¼ 2,900 Oe has induced the longitudinal electric field changenE3, and the lateral ME coefficient is a31¼nE3/nH1. From theaverage change in the displacement nu with and withoutmagnetic field, the electric field change can be obtained asnE3¼nu/d33D. From the displacement-voltage loop, withoutthe magnetic field, we could calculated the piezoelectriccoefficient d33 by d33¼ u/V. And V is the applied a.c. voltage(using 1 V here) which used to induce piezoelectric vibration inour experiment. By taking a mean nu value of 10.8 p.m. and amean u of 47.9 p.m., which are both measured in a remnant statewhen the d.c. voltage equals zero in the displacement-voltageloops, with the measured film thickness D of about 2.3 mm, the

–8 –6 –4 –2 0 2 4 6 8

–400

–200

0

200

400

H (kOe)

M (

emu

cc–1

)

In-plane

Out-of-plane

–80 –40 0 40 80

–80

–40

0

40

80

100 K 20 V100 K 60 V100 K 100 V

P (

μC c

m–2

)

E (kV mm–1)

Figure 3 | Ferroelectric and magnetic properties of nanocomposite films. (a) P–E hysteresis loops for the quasi-(0-3) BFO/BFO–CFO/BFO

nanocomposite film. (b) M–H hysteresis loops along the in-plane and out-of plane directions for the quasi-(0-3) BFO/BFO–CFO/BFO nanocomposite film.

The magnetization is normalized to the volume fraction of the CFO phase.





nE3 is calculated to be 9.8� 105 mV cm� 1, giving rise toa31¼ 338 mV cm� 1 Oe� 1. This value is several times higherthan those of the (0-3) type composite films reportedpreviously25,34, indicating enhanced atomic-scale tight bindingbetween the two constituting phases during the film growthprocess.

DiscussionThe unique heterostructure of the films can account for theobserved property shifts. The changes in the BFO piezoresponsewith magnetic bias applied across the CFO nanopillars demon-strate a strain-mediated ME coupling between magnetostrictiveand piezoelectric phases in the intermediate BFO–CFO layer.

–30 –20 –10 0 10 20 300

50

100

150

200

250With M

Pha

se (

degr

ee)

DC bias (V)

Without M

ba

dc

7 8 9 10 11 12 1330

40

50

60

70

Dis

plac

emen

t (pm

)

Coercive voltage (V)

Without MWith M

0 2 4 6 8 10 12 1430

40

50

60

70

Point number

Without MWith M

Dis

plac

emen

t (pm

)

–40 –20 0 20 400

10

20

30

40

50

60

With M

Without M

Dis

plac

emen

t (pm

)

DC bias (V)

Figure 4 | Piezoelectric properties and indirect ME effect of nanocomposite films. (a,b) The switching characteristics of piezoresponse for quasi-(0-3)

BFO/BFO–CFO/BFO nanocomposite film with and without the application of external magnetic field: the displacement-voltage butterfly loop (a)

and the phase-voltage hysteresis loop (b). (c,d) Statistical chart for the piezoresponse displacement value difference with and without the application

of external magnetic. The measured piezoresponse displacement as a function of each selected area (c) and the coercive field (d) in the same

measurement area. The external magnetic is adopted to be 2,900 Oe.

BFO

CFO H

E

[001]

[100] [010]

BFO

CFO

[001]

90

60

30

0

[100]

[010]

Δ�/ppm

Figure 5 | Phase-field simulation on the piezoelectric response of the nanocomposite film. (a) Schematics of the phase-field model. (b) Distribution of

the out-of-plane piezoelectric strain response enhancement De in the cross-section plane (as shown by the grey translucent plane in a) to an electric

voltage U¼ 20 V, after application of an external magnetic field along the in-plane [100] direction.





Upon applying an in-plane magnetic field, the CFO nanopillarsundergo a remarkable in-plane contraction and out-of-planeelongation due to the large negative magnetostriction constantl100 of � 590 p.p.m. (ref. 35). Given the good lattice coherencybetween the CFO nanopillar and BFO matrix (Fig. 2d,e), the in-plane compressive magnetostrictive strains can be effectivelytransferred to the BFO regions at close to the top and bottomsurfaces of the CFO pillar, which in turn expands the out-of-plane lattice parameters of the BFO therein. On the other hand,the out-of-plane tensile magnetostrictive strains can directlystretch the BFO regions along the lateral surfaces of the CFO.Both processes stabilize the polarization along the out-of-planeo1114 axes, which therefore accounts for the increase in theEc (Fig. 4b).

These magnetostrictive strains transferred from the CFO alsoaccount for the increase in the piezoresponse (Fig. 4a,c,d). Forclearer illustration, we performed phase-field simulation (see themodel setup in Fig. 5a and details in Methods) for the presentquasi-(0-3) nanocomposite heterostructure. Figure 5b shows thesimulated enhancement of the piezoelectric strain. The BFOregions along the lateral surfaces of the in-plane-magnetized CFOpillars are subjected to the out-of-plane tensile magnetostrictivestrains and gains polarizability along the out-of-plane direction,making primary contribution to the piezoresponse enhancement.On the other hand, although the BFO regions adjacent to the topand bottom surfaces of the CFO nanopillar also exhibit anappreciable enhancement in the out-of-plane piezoelectric strainresponse, the magnitude of enhancement is much smaller becauseof the smaller contacted surface area. Our simulation results alsosuggest an increase of 23.6% in the average out-of-planepiezoelectric strain from 165 to 204 p.p.m. on the application ofin-plane magnetic field, consistent with the experimentallyobserved 22.5% increase in the out-of-plane piezoelectricdisplacement (Fig. 4a).

Achieving a better coupling between the piezoelectric andmagnetostrictive components is a key point to obtain anenhanced ME effect in the ME composites. We have developeda novel growth strategy and fabricated a new architecturednanocomposite heterostructure by combining the structuraladvantages of the (2-2) and (1-3) phase connectivity. The phasedistribution in the heteroepitaxial films not only overcomes theclamping effect from substrate (inherent to (2-2) connectivity)but also significantly reduces the leakage current paths throughthe ferromagnetic phase (inherent to (1-3) connectivity). We alsoidentify an indirect ME effect by the switching characteristicsof the piezoresponse for the heterostructure, and demonstratethat the improved connectivity amongst the constituent phasesresults in a better coupling between the piezoelectric andmagnetostrictive phases, yielding notable ME effects. Such newquasi-(0-3) ME heterostructures and their combination ofproperties hold substantial promise for multifunctional micro-device applications.

MethodsSample growth. An epitaxial BiFeO3 bottom layer thin film was first grown on(001) oriented STO substrates by pulsed laser deposition. Two-phase 0.65BiFeO3–0.35CoFe2O4 thin films were thereafter deposited on top of BFO film as a middlelayer. This self-assembled layer was then used as a template, onto which a thirdBiFeO3 top layer was deposited. All three layers were deposited at 700 �C under a90 mTorr oxygen partial pressure. The films were deposited using a Lambda 305iKrF laser with a wavelength of 248 nm, focused to a spot size of 2 mm2, andincident on the surface of a target at an energy density of 3 J cm� 2. The distancebetween the substrate and target was 6 cm and the base vacuum of the chamber was10� 5 Torr.

Structure characterization. The crystal structures of thin films were determinedusing a Philips X’pert high resolution X-ray diffractometer equipped with atwo-bounce hybrid monochromator and an open three-circle Eulerian cradle. The

surface topology and piezoresponse of the BFO–CFO thin films were studied withMFP-3D atomic force microscopy (Asylum Research, Santa Barbara, CA). ThePFM studies were carried out on DART (dual AC resonance tracking) mode36 witha drive amplitude (AC voltage) of 1.5 V applied to the tip. The used conductivecantilever was Asylmelectric (Ti/Ir coating) with a resonance frequency of around70 kHz and a spring constant of 2 nN nm� 1. The contact resonant frequency in theDART mode was around 280 kHz. Cross-section thin-foil specimens for TEM andSTEM observations were prepared by cutting, grinding and dimpling the samplesdown to B20mm. In the Ar ion-beam thinning process, we applied a gun voltageof 1–4 kV and an incident beam angle of 4–6� to avoid radiation damage. SADP,EDS and TEM images were taken using a JEOL JEM-2010F operated at 200 kV.HAADF and ABF images were observed using a 200-kV ARM-200FC STEMequipped with a probe corrector (CEOS GmbH), providing an unprecedentedopportunity to probe structures with sub-Ångstrom resolution. For the HAADFimaging, a probe convergence angle of B22 mrad and a detector with an innersemiangle of over 60 mrad were adopted. The ABF STEM images were taken usinga detector of 6–25 mrad.

Electrical and magnetic measurements. Ferroelectric P–E hysteresis loops weremeasured by a modified Sawyer-Tower circuit. Magnetic hysteresis loops weremeasured using a vibrating sample magnetometer (VSM 7304, Lake Shore Cryo-tronics, Westerville, OH). The coupling of electric and magnetic measurementswere done on a MFP-3D (Asylum Research) AFM equipped with one externalvaried field magnet (VFM, Asylum Research). The sample was placed on the VFMmodule that was compatible to the AFM scanner. For the point piezoresponsemeasurement, a triangle–square waveform with the amplitude of 30 V was appliedto the tip, accompanying with 1 V AC bias. After the switching curve was achieved,the in-plane magnetic field was increased to 2,900 Oe, followed by the samemeasurement of the same domain.

Phase-field simulation. The total size of the simulation system was 750�750� 1,300 nm3, which was discretized into a three-dimensional array of150� 150� 260 cells. A cylindrical CFO particle with diameter d¼ 500 nm andheight h¼ 300 nm in the middle of a BFO matrix (Fig. 5a) together with periodicboundaries was considered. Temporal evolution of the FE polarization in BFO andthe magnetization in CFO was simulated by solving the time-dependent GinzburgLandau equation37 and the Landau–Lifshitz–Gilbert equation using a semi-implicitFourier-spectral method38. The strain distribution in the nanocomposite systemwas obtained by solving the elastic equilibrium equation39

@ cijkl ekl � e0kl

� �� @xj ¼ 0; ð1Þ

with the stress-free strain e0 generated by electrostriction and magnetostrictioncalculated as e0

ij ¼ QijklPkPl and

e0ij ¼

32l100 mimj� 1

3

� �; i ¼ j

32l111mimj; i 6¼ j

�ð2Þ

for the BFO and CFO phases, respectively; Q and l were electrostrictivecoefficient of BFO and saturation magnetostriction of CFO, respectively, whichwere taken as Q11¼ 0.032 C� 2 m4, Q12¼ � 0.016 C� 2 m4, Q44¼ 0.020 C� 2 m4,l100¼ � 590 p.p.m. and l111¼ 120 p.p.m. (ref. 35).

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AcknowledgementsY.X.L, J.F.L. and D.V. thank the financial supports from Air Force Office of ScientificResearch (FA9550-09-1-0552) in USA. Z.C.W. thanks the financial supports from theGrant-in-Aid for Young Scientists (A) (grant no. 24686069), the National NaturalScience Foundation of China (NSFC) (grant no. 11332013), the JSPS and CAS underJapan-China Scientific Cooperation Program, and the Murata Science Foundation (grantno. H26-087). T.N.Y., J.M.H. and L.Q.C. thank the financial support from the NationalScience Foundation (NSF) under the grant no. of DMR 140714(Chen). We thank Y. J.Wang, C. T. Luo and R. Viswan at the Virginia Tech and Y. D. Yang at Xi’an JiaotongUniversity for useful discussions.

Author contributionsY.X.L. conceived this work; Y.X.L., Z.C.W. and J.J.Y. designed the experiments; Y.X.L.,Z.C.W., J.J.Y., C.L.C. and R.S. performed the experiments; T.N.Y., J.M.H. and L.Q.C.performed phase-field simulations; Y.X.L., Z.C.W., J.J.Y., Z.G.W., C.L.C., R.S., T.N.Y. andJ.M.H. analysed the data; Y.X.L., Z.C.W., T.N.Y and J.M.H. wrote the manuscript withD.V.’s assistance. All authors discussed the results and commented on the manuscript.

Additional informationSupplementary Information accompanies this paper at http://www.nature.com/naturecommunications.

Competing financial interests: The authors declare no competing financial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions.

How to cite this article: Li, Y. et al. Magnetoelectric quasi-(0-3) nanocompositeheterostructures. Nat. Commun. 6:6680 doi: 10.1038/ncomms7680 (2015).






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